Adaptive Antenna Tuning Based On Measured Antenna Impedance

ABSTRACT

A system includes an antenna, an impedance measurement circuit, an impedance tuning circuit, and a controller. The impedance measurement circuit can include a test current source that conveys a test current through the antenna, and a voltage sensor that measure a voltage across the antenna while the test current is conveyed through the antenna. The impedance tuning circuit can be coupled to the antenna leads and can include one or more reactive elements that can be selectively coupled to the antenna, or otherwise adjusted, to effect adjustment of the impedance connected to the antenna. The controller can: (i) use the impedance measurement circuit to obtain a measurement indicative of an impedance of the antenna; (ii) determine an adjustment to the impedance tuning circuit based on the obtained measurement; and (iii) cause the impedance tuning circuit to make the determined adjustment.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

In order to maximize the energy/power extracted from or input to anantenna the impedance of the circuits connected to the antenna should bematched to the impedance of the antenna. Otherwise, the signal/energytransfer between the antenna and the connected circuitry is degraded. Ifthe antenna is used in a wireless power link this means reduced powerwill be transferred from the antenna to the power harvestingelectronics. If the antenna is used in a communication link this meansreduced signal-to-noise ratio for the signal transferred from theantenna to the receiver.

SUMMARY

A system includes an antenna, an impedance measurement circuit, animpedance tuning circuit, and a controller. The impedance measurementcircuit can include a test current source that conveys a test currentthrough the antenna, and a voltage sensor that measure a voltage acrossthe antenna while the test current is conveyed through the antenna. Theimpedance tuning circuit can be coupled to the antenna leads and caninclude one or more reactive elements that can be selectively coupled tothe antenna, or otherwise adjusted, to effect adjustment of theimpedance connected to the antenna. The controller can: (i) use theimpedance measurement circuit to obtain a measurement indicative of animpedance of the antenna; (ii) determine an adjustment to the impedancetuning circuit based on the obtained measurement; and (iii) cause theimpedance tuning circuit to make the determined adjustment.

Some embodiments of the present disclosure provide a system. The systemcan include an antenna, an impedance measurement circuit, an impedancetuning circuit, and a controller. The impedance measurement circuit canbe electrically coupled to the antenna. The impedance measurementcircuit can be configured to be used to obtain a measurement indicativeof an impedance of the antenna. The impedance tuning circuit can beelectrically coupled to the antenna. The impedance tuning circuit caninclude one or more reactive elements that can be used to adjust theimpedance coupled to the antenna. The controller can be configured to:(i) use the impedance measurement circuit to obtain a measurementindicative of an impedance of the antenna; (ii) determine an adjustmentto the impedance tuning circuit based on the obtained measurement; and(iii) cause the impedance tuning circuit to make the determinedadjustment.

Some embodiments of the present disclosure provide a method. The methodcan include obtaining a measurement indicative of an impedance of anantenna. The method can include determining, based on the obtainedmeasurement, an adjustment to an impedance tuning circuit coupled to theantenna. The impedance tuning circuit includes one or more reactiveelements that can be used to adjust the impedance coupled to theantenna. The method can include causing the impedance tuning circuit tomake the determined adjustment.

Some embodiments of the present disclosure provide a body-mountabledevice including a polymeric material, a substrate, an antenna, animpedance measurement circuit, an impedance tuning circuit, and acontroller. The polymeric material can be formed to include abody-mountable surface. The substrate can be at least partially embeddedwithin the polymeric material. The antenna can be disposed on thesubstrate. The impedance measurement circuit can be disposed on thesubstrate. The impedance measurement circuit can be coupled to theantenna. The impedance measurement circuit can be configured to be usedto obtain a measurement indicative of an impedance of the antenna. Theimpedance tuning circuit can be disposed on the substrate. The impedancetuning circuit is coupled to the antenna. The impedance tuning circuitcan include one or more reactive elements that can be used to adjust theimpedance coupled to the antenna. The controller can be disposed on thesubstrate. The controller can be configured to: (i) use the impedancemeasurement circuit to obtain a measurement indicative of an impedanceof the antenna; (ii) determine an adjustment to the impedance tuningcircuit based on the obtained measurement; and (iii) cause the impedancetuning circuit to make the determined adjustment.

Some embodiments of the present disclosure provide means for obtaining ameasurement indicative of an impedance of an antenna. Some embodimentsof the present disclosure provide means for determining, based on theobtained measurement, an adjustment to an impedance tuning circuitcoupled to the antenna. The impedance tuning circuit includes one ormore reactive elements that can be used to adjust the impedance coupledto the antenna. Some embodiments of the present disclosure provide meansfor causing the impedance tuning circuit to make the determinedadjustment.

These as well as other aspects, advantages, and alternatives, willbecome apparent to those of ordinary skill in the art by reading thefollowing detailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example system that includes abody-mountable device in wireless communication with an external reader.

FIG. 2A is a top view of an example eye-mountable device.

FIG. 2B is a side view of the example eye-mountable device shown in FIG.2A.

FIG. 3 is a functional block diagram of an example system configured toadjust for temporal variations in antenna impedance.

FIG. 4A is a functional block diagram of an example system configured toadjust for temporal variations in antenna impedance.

FIG. 4B is a functional block diagram of an example system configured toadjust for temporal variations in antenna impedance.

FIG. 5A is a block diagram of an example system with an eye-mountabledevice and an external reader.

FIG. 5B is a block diagram of the eye-mountable device shown in FIG. 5A.

FIG. 6A illustrates an example head-mountable eyeglass frame, accordingto an example embodiment.

FIG. 6B illustrates the example head-mountable eyeglass frame of FIG. 6Ain a head-mounted configuration.

FIG. 6C is a close-in view of FIG. 6B enhanced to show an exampleeye-mountable device mounted on a cornea and the loop antenna disposedon the example head-mountable eyeglass frame.

FIG. 7 is a flowchart of an example process involving adjusting animpedance tuning circuit based on a real time measurement of theimpedance of an antenna.

FIG. 8 depicts a computer-readable medium configured according to anexample embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying figures, which form a part hereof. In the figures, similarsymbols typically identify similar components, unless context dictatesotherwise. The illustrative embodiments described in the detaileddescription, figures, and claims are not meant to be limiting. Otherembodiments may be utilized, and other changes may be made, withoutdeparting from the scope of the subject matter presented herein. It willbe readily understood that the aspects of the present disclosure, asgenerally described herein, and illustrated in the figures, can bearranged, substituted, combined, separated, and designed in a widevariety of different configurations, all of which are explicitlycontemplated herein.

I. Overview

A body-mountable electronics platform can include bio-interactiveelectronics, such as sensors and the like, and an antenna to facilitatecommunication with an external reader. The body-mountable electronicsplatform can also harvest energy from incident radiation to power theelectronics. In some cases, incident light may energize photovoltaiccells. Additionally or alternatively, power can be provided by incidentradio frequency radiation inductively harvested using the antenna. Arectifier and/or regulator can be incorporated with the controlelectronics to generate a DC voltage to power the electronics from theharvested energy. The antenna can be arranged as a loop of conductivematerial with leads connected to the control electronics. In someembodiments, such a loop antenna can also wirelessly communicate thesensor readings to an external reader by modifying the backscatterradiation from the antenna in a manner that can be detected by thereader.

One such body-mountable electronic device may be an eye-mountable deviceformed of a polymeric material that is shaped to be contact mounted toan eye, similar to a contact lens. A substrate embedded within thepolymeric material can be used to mount bio-interactive electronics andassociated power and communication electronics. In one example, anantenna disposed on the substrate is used to harvest energy fromincident radio frequency radiation, and the harvested energy can beused, via a rectifier and voltage regulator, to power the remainingelectronics. Communication electronics can be used to modulate theimpedance of the energy-harvesting antenna to cause correspondingmodifications of the antenna's backscatter radiation, which can then bedetected by a reader.

The impedance exhibited by the antenna may change over time due tovarious factors. Changes in antenna impedance may occur for example, dueto variations in environmental temperature, humidity, antenna geometry(e.g., due to flexing), and/or other changes in the near field of theantenna, such as distributions of dielectric materials. For instance, inbody-mountable and/or implantable electronic devices, an antenna may beat least partially surrounded by variable distributions of tissue and/orbodily fluids. As a result, the antenna impedance will change fromperson to person (e.g., due to different body characteristics), and alsoover time for a given person (e.g., due to changes in mountinglocation). Changing the distribution of physiological material (tissue,fluid, etc.) changes the dielectric loading of the antenna, and therebychanges the impedance exhibited by the antenna. In an eye-mountabledevice, dielectric variability may be due to variations in cornealthickness, relative position of eyelids, distribution of tear fluid, andso forth.

To ensure efficient power transfer to/from an antenna, the drivingtransceiver circuitry can be conjugate matched to the antenna impedance.Conventional approaches to tuning an antenna may provide a fixedimpedance adjustment via a resonant tuning circuit, but such approachesare ill-suited to antennas that exhibit temporal variations inimpedance. Conventional approaches can therefore lead to impedancemismatches between the antenna and its connected circuitry. The mismatchwill result in reduced power transfer in a wireless power link andreduced achievable data rate in a wireless data link.

Systems disclosed herein provide for dynamically adjusting an adjustableimpedance circuit to account for changes in the antenna impedance overtime. An impedance measurement circuit first measures the impedance ofthe antenna, and then the measurement is used as a basis to adjust animpedance tuning circuit as needed. The impedance matching circuit issituated between the antenna and the transceiver and/or energyharvesting circuitry that receives signals from the antenna. Theimpedance tuning circuit can have multiple reactive elements (e.g.,inductors and/or capacitors) that can be selectively connected ordisconnected from the antenna. Connecting the reactive components indifferent configurations thereby adjusts the impedance of the tuningcircuit. In some cases, the impedance tuning circuit may additionally oralternatively include one or more components with adjustable reactances(e.g., a varactor that exhibits a capacitance dependent on an appliedvoltage). The impedance tuning circuit can be adjusted such that totalimpedance of the circuitry connected to the antenna (i.e., thetransceiver and the tuning circuit) is conjugate matched to the measuredantenna impedance. The measurement and adjustment operations can beperformed iteratively to dynamically tune the impedance of theantenna-connected circuitry in accordance with the dynamically varyingantenna impedance.

A measurement circuit can be used for obtaining a measurement of theantenna impedance. The measurement circuit can include a current sourcethat sends a test current through the antenna at a desired frequency,and a voltage sensor that measures the voltage across the antenna inresponse to the current. The output from the voltage sensor provides anindication of the voltage waveform, which can then be measured andconverted to digital values (e.g., using an analog-to-digitalconverter). The test current can also be measured to provide anindication of the current waveform. In some cases, the test current maybe measured through a current mirror, which allows for measuring themirrored test current without loading the antenna. To measure themirrored test current, a transimpedance amplifier may be connected tothe current mirror, and the output can be measured and converted todigital values using an analog-to-digital converter.

Moreover, a single analog-to-digital converter may be used to generatedigital values for both the voltage measurements and the currentmeasurements. A multiplexer may be used to connect the singleanalog-to-digital converter to either the voltage amplifier or thetransimpedance amplifier.

The test current and resulting voltage can each be measured to obtaindata over several cycles at the frequency of interest (i.e., thefrequency of the test current), and a digital controller can be used todetermine an impedance adjustment to make based on the measurements. Thedigital controller can then cause the impedance tuning circuit to makethe determined adjustment, such as by setting control lines to turntransistors on or off and thereby selectively connect capacitive and/orinductive components to the tuning circuit. Furthermore, in someexamples, the adjusting to the impedance tuning circuit may be regulatedby an analog circuit that functions to generate suitable control signalsto the impedance tuning circuit based on the impedance of the antenna.

In some examples, the digital controller (or a hardware equivalent) maydetermine phase information of the antenna impedance using quadraticdown-conversions of the sampled waveforms. For instance, the real andimaginary contributions to the impedance (e.g., resistive and reactivecontributions) can be separately extracted from the obtained voltage andcurrent measurements. The phase can then be calculated as the arctangentof the ratio of the magnitude of the imaginary component to themagnitude of the real component. In some cases, the conjugate matchingprocedure may proceed by only adjusting the reactive contribution to theimpedance.

II. Example Body-Mountable Electronics Platform

FIG. 1 is a block diagram of a system 100 that includes a body-mountabledevice 110 in wireless communication with an external reader 180. Theexposed regions of the body-mountable device 110 are made of a polymericmaterial 120 formed to be contact-mounted to a corneal surface of aneye. A substrate 130 is embedded in the polymeric material 120 toprovide a mounting surface for a power supply 140, a controller 150,sensor electronics 160, and a communication antenna 170. The sensorelectronics 160 are operated by the controller 150. The power supply 140supplies operating voltages to the controller 150 and/or the sensorelectronics 160. The antenna 170 is operated by the controller 150 tocommunicate information to and/or from the body-mountable device 110.The antenna 170, the controller 150, the power supply 140, and thesensor electronics 160 can all be situated on the embedded substrate130. Because the body-mountable device 110 includes electronics and isconfigured to be contact-mounted to an eye, it is also referred toherein as an ophthalmic electronics platform.

To facilitate contact-mounting, the polymeric material 120 can have aconcave surface configured to adhere (“mount”) to a moistened cornealsurface (e.g., by capillary forces with a tear film coating the cornealsurface). Additionally or alternatively, the device 110 can be adheredby a vacuum force between the corneal surface and the polymeric material120 due to the concave curvature. While mounted with the concave surfaceagainst the eye, the outward-facing surface of the polymeric material120 can have a convex curvature that is formed to not interfere witheye-lid motion while the body-mountable device 110 is mounted to theeye. For example, the polymeric material 120 can be a substantiallytransparent curved polymeric disk shaped similarly to a visioncorrection contact lens.

The polymeric material 120 can include one or more biocompatiblematerials, such as those employed for use in contact lenses or otherophthalmic applications involving direct contact with the cornealsurface. The polymeric material 120 can optionally be formed in partfrom such biocompatible materials or can include an outer coating withsuch biocompatible materials. The polymeric material 120 can includematerials configured to moisturize the corneal surface, such ashydrogels and the like. In some embodiments, the polymeric material 120can be a deformable (“non-rigid”) material to enhance wearer comfort. Insome embodiments, the polymeric material 120 can be shaped to provide apredetermined, vision-correcting optical power, such as can be providedby a contact lens. Moreover, the polymeric material 120 may be formed tofacilitate mounting to another body surface, such as a tooth surface,ear surface, skin surface, etc., and the polymeric material 120 may haveproperties (e.g., flexibility, surface hardness, lubricity, etc.)selected to be suitable for such mounting locations.

The substrate 130 includes one or more surfaces suitable for mountingthe sensor electronics 160, the controller 150, the power supply 140,and the antenna 170. The substrate 130 can be employed both as amounting platform for chip-based circuitry (e.g., by flip-chip mountingto connection pads) and/or as a platform for patterning conductivematerials (e.g., gold, platinum, palladium, titanium, copper, aluminum,silver, metals, other conductive materials, combinations of these, etc.)to create electrodes, interconnects, connection pads, antennae, etc. Insome embodiments, substantially transparent conductive materials (e.g.,indium tin oxide) can be patterned on the substrate 130 to formcircuitry, electrodes, etc. For example, the antenna 170 can be formedby forming a pattern of gold or another conductive material on thesubstrate 130 by deposition, photolithography, electroplating, etc.Similarly, interconnects 151, 157 between the controller 150 and thesensor electronics 160, and between the controller 150 and the antenna170, respectively, can be formed by depositing suitable patterns ofconductive materials on the substrate 130. A combination ofmicrofabrication techniques including, without limitation, the use ofphotoresists, masks, deposition techniques, and/or plating techniquescan be employed to pattern materials on the substrate 130.

The substrate 130 can be a relatively rigid material, such aspolyethylene terephthalate (“PET”), parylene, or another materialconfigured to structurally support the circuitry and/or chip-basedelectronics within the polymeric material 120. The body-mountable device110 can alternatively be arranged with a group of unconnected substratesrather than a single substrate. For example, the controller 150 and asensor in sensor electronics 160 can be mounted to one substrate, whilethe antenna 170 is mounted to another substrate and the two can beelectrically connected via the interconnects 157. In another example,the substrate 130 can include separate partitions that each supportseparated, overlapped coiled portions of the antenna 170. Such as, forexample, an example in which the antenna 170 is divided into multiplewindings that wrap around the body-mountable device 110circumferentially at respective radii, and are connected in paralleland/or in series. To facilitate movement of the individual windings withrespect to one another, and thereby enhance flexibility of thebody-mountable device 110, and help prevent binding, etc., theindividual windings may each be mounted on divided portions of thesubstrate 130, which may substantially correspond to the windings ofsuch an antenna.

The substrate 130 has a width sufficient to provide a mounting platformfor the embedded electronics components. The substrate 130 can have athickness sufficiently small to allow the substrate 130 to be embeddedin the polymeric material 120 without influencing the profile of thebody-mountable device 110. The substrate 130 can have a thicknesssufficiently large to provide structural stability suitable forsupporting the electronics mounted thereon. For example, in animplementation in which the body-mountable device 110 is aneye-mountable device, the substrate 130 can be shaped as a ring with adiameter of about 10 millimeters, a radial width of about 1 millimeter(e.g., an outer radius 1 millimeter larger than an inner radius), and athickness of about 50 micrometers. The substrate 130 can optionally bealigned with the curvature of an eye-mountable surface of the polymericmaterial 120 (e.g., the convex or concave surfaces). For example, thesubstrate 130 can be shaped along the surface of an imaginary conebetween two circular segments that define an inner radius and an outerradius. In such an example, the surface of the substrate 130 along thesurface of the imaginary cone defines an inclined surface that isapproximately aligned with the curvature of the eye mounting surface(concave) and/or outward surface (convex) at that radius.

The power supply 140 is configured to harvest ambient energy to powerthe controller 150 and sensor electronics 160. For example, aradio-frequency energy-harvesting antenna 142 can capture energy fromincident radio radiation. Additionally or alternatively, solar cell(s)144 (“photovoltaic cells”) can capture energy from incoming ultraviolet,visible, and/or infrared radiation. Furthermore, an inertial powerscavenging system can be included to capture energy from ambientvibrations. The energy harvesting antenna 142 can optionally be adual-purpose antenna that is also used to communicate information to theexternal reader 180. That is, the functions of the communication antenna170 and the energy harvesting antenna 142 can be accomplished with thesame physical antenna.

A rectifier/regulator 146 can be used to condition the captured energyto a stable DC supply voltage 141 that is supplied to the controller150. For example, the energy harvesting antenna 142 can receive incidentradio frequency radiation. Varying electrical signals on the leads ofthe antenna 142 are output to the rectifier/regulator 146. Therectifier/regulator 146 rectifies the varying electrical signals to a DCvoltage and regulates the rectified DC voltage to a level suitable foroperating the controller 150. Additionally or alternatively, outputvoltage from the solar cell(s) 144 can be regulated to a level suitablefor operating the controller 150. The rectifier/regulator 146 caninclude one or more energy storage devices to mitigate high frequencyvariations in the ambient energy gathering antenna 142 and/or solarcell(s) 144. For example, one or more energy storage devices (e.g., acapacitor, a battery, etc.) can be connected in parallel across theoutputs of the rectifier 146 to regulate the DC supply voltage 141 andconfigured to function as a low-pass filter.

The controller 150 can be turned on when the DC supply voltage 141 isprovided to the controller 150, and the logic in the controller 150 canthen operate the sensor electronics 160 and the antenna 170. Thecontroller 150 can include logic circuitry configured to operate thesensor electronics 160 so as to sense a characteristic of theenvironment of the body-mountable device 110. For example, the sensorelectronics 160 may include an analyte bio-sensor 162 configured tosense an analyte in the biological environment (e.g., tear film) of thebody-mountable device 110. Additionally or alternatively, the sensorelectronics 160 could include a light sensor 164 that is configured todetect an intensity of incident light, or perhaps an image sensorconfigured to capture an image from a perspective of the body-mountabledevice 110 (e.g., an external environment outside of the eye or aninternal environment within the eye).

In one example, the controller 150 includes a bio-sensor interfacemodule 152 that is configured to operate analyte bio-sensor 162. Theanalyte bio-sensor 162 can be, for example, an amperometricelectrochemical sensor that includes a working electrode and a referenceelectrode. A voltage can be applied between the working and referenceelectrodes to cause an analyte to undergo an electrochemical reaction(e.g., a reduction and/or oxidation reaction) at the working electrode.The electrochemical reaction can generate an amperometric current thatcan be measured through the working electrode. The amperometric currentcan be dependent on the analyte concentration. Thus, the amount of theamperometric current that is measured through the working electrode canprovide an indication of analyte concentration. In some embodiments, thebio-sensor interface module 152 can be a potentiostat configured toapply a voltage difference between working and reference electrodeswhile measuring a current through the working electrode.

In some instances, a reagent can also be included to sensitize theelectrochemical sensor to one or more desired analytes. For example, alayer of glucose oxidase (“GOx”) proximal to the working electrode cancatalyze glucose oxidation to generate hydrogen peroxide (H₂O₂). Thehydrogen peroxide can then be electro-oxidized at the working electrode,which releases electrons to the working electrode, resulting in anamperometric current that can be measured through the working electrode.

The current generated by either reduction or oxidation reactions isapproximately proportionate to the reaction rate. Further, the reactionrate is dependent on the rate of analyte molecules reaching theelectrochemical sensor electrodes to fuel the reduction or oxidationreactions, either directly or catalytically through a reagent. In asteady state, where analyte molecules diffuse to the electrochemicalsensor electrodes from a sampled region at approximately the same ratethat additional analyte molecules diffuse to the sampled region fromsurrounding regions, the reaction rate is approximately proportionate tothe concentration of the analyte molecules. The current measured throughthe working electrode thus provides an indication of the analyteconcentration.

The controller 150 can optionally include a display driver module 154for operating a pixel array 164. The pixel array 164 can be an array ofseparately programmable light transmitting, light reflecting, and/orlight emitting pixels arranged in rows and columns. The individual pixelcircuits can optionally include liquid crystal technologies,microelectromechanical technologies, emissive diode technologies, etc.to selectively transmit, reflect, and/or emit light according toinformation from the display driver module 154. Such a pixel array 164can also optionally include more than one color of pixels (e.g., red,green, and blue pixels) to render visual content in color. The displaydriver module 154 can include, for example, one or more data linesproviding programming information to the separately programmed pixels inthe pixel array 164 and one or more addressing lines for setting groupsof pixels to receive such programming information. Such a pixel array164 situated on the eye can also include one or more lenses to directlight from the pixel array to a focal plane perceivable by the eye.

The controller 150 can also include a communication circuit 156 forsending and/or receiving information via the antenna 170. Thecommunication circuit 156 can optionally include one or moreoscillators, mixers, frequency injectors, etc. to modulate and/ordemodulate information on a carrier frequency to be transmitted and/orreceived by the antenna 170. In some examples, the body-mountable device110 is configured to indicate an output from a bio-sensor, light sensor,and/or image sensor by modulating an impedance of the antenna 170 in amanner that can be perceived by the external reader 180. For example,the communication circuit 156 can cause variations in the amplitude,phase, and/or frequency of backscatter radiation from the antenna 170,and such variations can be detected by the reader 180.

The controller 150 is connected to the sensor electronics 160 viainterconnects 151. For example, where the controller 150 includes logicelements implemented in an integrated circuit to form the bio-sensorinterface module 152 and/or light sensor interface 154, a patternedconductive material (e.g., gold, platinum, palladium, titanium, copper,aluminum, silver, metals, combinations of these, etc.) can connect aterminal on the chip to the sensor electronics 160. Similarly, thecontroller 150 is connected to the antenna 170 via interconnects 157.

In some embodiments, the interconnects 157 may further include animpedance tuning circuit that connects the antenna 170 to the controller150. The impedance tuning circuit may function to selectively connectreactive components to the antenna 170 so as to adjust the impedance ofcircuit components connected to the antenna 170 to be conjugate matchedwith the impedance of the antenna 170. In particular, the combinedimpedance of the interconnects 157 (including the tuning circuitcomponents) and the communication circuit 156 (e.g., a transceiver) canbe conjugate matched to the impedance of the antenna 170. Moreover, thematching procedure can be based on real time measurements of the antennaimpedance so as to account for variations in the antenna impedance,which may occur due to variable near field conditions for antennassituated in body-mountable devices. To make impedance adjustments basedon the real time impedance of the antenna 170, an impedance measurementcircuit can also be associated with the controller 150 and/orinterconnects 157. The impedance measurement circuit can provide a testcurrent through the antenna 170, and measure a resulting voltage acrossthe antenna 170 to ascertain the impedance. Upon determining theimpedance of the antenna 170, an adjustment to the impedance tuningcircuit that can match impedances can be determined, and the tuningcircuit can be adjusted accordingly.

It is noted that the block diagram shown in FIG. 1 is described inconnection with functional modules for convenience in description.However, embodiments of the body-mountable device 110 can be arrangedwith one or more of the functional modules (“sub-systems”) implementedin a single chip, integrated circuit, and/or physical component. Forexample, while the rectifier/regulator 146 is illustrated in the powersupply block 140, the rectifier/regulator 146 can be implemented in achip that also includes the logic elements of the controller 150 and/orother features of the embedded electronics in the body-mountable device110. Thus, the DC supply voltage 141 that is provided to the controller150 from the power supply 140 can be a supply voltage that is providedto components on a chip by rectifier and/or regulator components locatedon the same chip. That is, the functional blocks in FIG. 1 shown as thepower supply block 140 and controller block 150 need not be implementedas physically separated modules. Moreover, one or more of the functionalmodules described in FIG. 1 can be implemented by separately packagedchips electrically connected to one another.

Additionally or alternatively, the energy harvesting antenna 142 and thecommunication antenna 170 can be implemented with the same physicalantenna. For example, a loop antenna can both harvest incident radiationfor power generation and communicate information via backscatterradiation.

The external reader 180 includes an antenna 188 (or a group of multipleantennas) to send and receive wireless signals 171 to and from thebody-mountable device 110. The external reader 180 also includes acomputing system with a processor 186 in communication with a memory182. The memory 182 is a non-transitory computer-readable medium thatcan include, without limitation, magnetic disks, optical disks, organicmemory, and/or any other volatile (e.g., RAM) or non-volatile (e.g.,ROM) storage system readable by the processor 186. The memory 182 caninclude a data storage 183 to store indications of data, such as sensorreadings (e.g., from the analyte bio-sensor 162 and/or light sensor164), program settings (e.g., to adjust behavior of the body-mountabledevice 110 and/or external reader 180), etc. The memory 182 can alsoinclude program instructions 184 for execution by the processor 186 tocause the external reader 180 to perform processes specified by theinstructions 184. For example, the program instructions 184 can causeexternal reader 180 to communicate with the body-mountable device 110.The program instructions 184 can also cause the external reader 180 toprovide a user interface that allows for retrieving informationcommunicated from the body-mountable device 110 (e.g., sensor outputsfrom the analyte bio-sensor 162 and/or light sensor 164). The externalreader 180 can also include one or more hardware components foroperating the antenna 188 to send and receive the wireless signals 171to and from the body-mountable device 110. For example, oscillators,frequency injectors, encoders, decoders, amplifiers, filters, etc. candrive the antenna 188.

The external reader 180 can be a smart phone, digital assistant, orother portable computing device with wireless connectivity sufficient toprovide the wireless communication link 171. The external reader 180 canalso be implemented as an antenna module that can be plugged in to aportable computing device, such as in an example where the communicationlink 171 operates at carrier frequencies not commonly employed inportable computing devices. In some instances, the external reader 180is a special-purpose device configured to be worn relatively near awearer's eye to allow the wireless communication link 171 to operatewith a low power budget. For example, the external reader 180 can beintegrated in a piece of jewelry such as a necklace, earing, etc. orintegrated in an article of clothing or an accessory worn near the head,such as a hat, headband, a scarf, a pair of eyeglasses, etc.

In some embodiments, the system 100 can operate to non-continuously(“intermittently”) supply energy to the body-mountable device 110 topower the controller 150 and sensor electronics 160. For example, radiofrequency radiation 171 can be supplied to power the body-mountabledevice 110 long enough to operate the sensor electronics 160 andcommunicate an outcome of such operation. In such an example, thesupplied radio frequency radiation 171 can be considered aninterrogation signal from the external reader 180 to the body-mountabledevice 110 to request feedback (e.g., a sensor measurement). Byperiodically interrogating the body-mountable device 110 (e.g., bysupplying radio frequency radiation 171 to temporarily turn the deviceon), the external reader 180 can accumulate a set of measurements (orother feedback) over time from the sensor electronics 160 withoutcontinuously powering the body-mountable device 110.

FIG. 2A is a top view of an example eye-mountable electronic device 210(or ophthalmic electronics platform). FIG. 2B is an aspect view of theexample eye-mountable electronic device shown in FIG. 2A. It is notedthat relative dimensions in FIGS. 2A and 2B are not necessarily toscale, but have been rendered for purposes of explanation only indescribing the arrangement of the example eye-mountable electronicdevice 210. The eye-mountable device 210 is formed of a polymericmaterial 220 shaped as a curved disk. The polymeric material 220 can bea substantially transparent material to allow incident light to betransmitted to the eye while the eye-mountable device 210 is mounted tothe eye. The polymeric material 220 can be a biocompatible materialsimilar to those employed to form vision correction and/or cosmeticcontact lenses in optometry, such as polyethylene terephthalate (“PET”),polymethyl methacrylate (“PMMA”), polyhydroxyethylmethacrylate(“polyHEMA”), silicone hydrogels, combinations of these, etc. Thepolymeric material 220 can be formed with one side having a concavesurface 226 suitable to fit over a corneal surface of an eye. Theopposite side of the disk can have a convex surface 224 that does notinterfere with eyelid motion while the eye-mountable device 210 ismounted to the eye. A circular outer side edge 228 connects the concavesurface 224 and convex surface 226.

The eye-mountable device 210 can have dimensions similar to a visioncorrection and/or cosmetic contact lenses, such as a diameter ofapproximately 1 centimeter, and a thickness of about 0.1 to about 0.5millimeters. However, the diameter and thickness values are provided forexample purposes only. In some embodiments, the dimensions of theeye-mountable device 210 can be selected according to the size and/orshape of the corneal surface of the wearer's eye and/or to accommodateone or more components embedded in the polymeric material 220.

The polymeric material 220 can be formed with a curved shape in avariety of ways. For example, techniques similar to those employed toform vision-correction contact lenses, such as heat molding, injectionmolding, spin casting, etc. can be employed to form the polymericmaterial 220. While the eye-mountable device 210 is mounted in an eye,the convex surface 224 faces outward to the ambient environment whilethe concave surface 226 faces inward, toward the corneal surface. Theconvex surface 224 can therefore be considered an outer, top surface ofthe eye-mountable device 210 whereas the concave surface 226 can beconsidered an inner, bottom surface. The “top” view shown in FIG. 2A isfacing the convex surface 224 From the top view shown in FIG. 2A, theouter periphery 222, near the outer circumference of the curved disk iscurved to extend into the page, whereas the central region 221, near thecenter of the disk is curved to extend out of the page.

A substrate 230 is embedded in the polymeric material 220. The substrate230 can be embedded to be situated along the outer periphery 222 of thepolymeric material 220, away from the central region 221. The substrate230 does not interfere with vision because it is too close to the eye tobe in focus and is positioned away from the central region 221 whereincident light is transmitted to the eye-sensing portions of the eye.Moreover, the substrate 230 can be formed of a transparent material tofurther mitigate effects on visual perception.

The substrate 230 can be shaped as a flat, circular ring (e.g., a diskwith a centered hole). The flat surface of the substrate 230 (e.g.,along the radial width) is a platform for mounting electronics such aschips (e.g., via flip-chip mounting) and for patterning conductivematerials (e.g., via microfabrication techniques such asphotolithography, deposition, plating, etc.) to form electrodes,antenna(e), and/or interconnections. The substrate 230 and the polymericmaterial 220 can be approximately cylindrically symmetric about a commoncentral axis. The substrate 230 can have, for example, a diameter ofabout 10 millimeters, a radial width of about 1 millimeter (e.g., anouter radius 1 millimeter greater than an inner radius), and a thicknessof about 50 micrometers. However, these dimensions are provided forexample purposes only, and in no way limit the present disclosure. Thesubstrate 230 can be implemented in a variety of different form factors,similar to the discussion of the substrate 130 in connection with FIG. 1above.

A loop antenna 270, controller 250, and sensor electronics 260 aredisposed on the embedded substrate 230. The controller 250 can be a chipincluding logic elements configured to operate the sensor electronics260 and the loop antenna 270. The controller 250 is electricallyconnected to the loop antenna 270 by interconnects 257 also situated onthe substrate 230. Similarly, the controller 250 is electricallyconnected to the sensor electronics 260 by an interconnect 251. Theinterconnects 251, 257, the loop antenna 270, and any conductiveelectrodes (e.g., for an electrochemical analyte sensor, etc.) can beformed from conductive materials patterned on the substrate 230 by aprocess for precisely patterning such materials, such as deposition,photolithography, etc. The conductive materials patterned on thesubstrate 230 can be, for example, gold, platinum, palladium, titanium,carbon, aluminum, copper, silver, silver-chloride, conductors formedfrom noble materials, other metals, combinations of these, etc.

The loop antenna 270 is a layer of conductive material patterned alongthe flat surface of the substrate to form a flat conductive ring. Insome examples, to allow additional flexibility along the curvature ofthe polymeric material, the loop antenna 270 can include multiplesubstantially concentric sections electrically joined together inparallel or in series. Each section can then flex independently alongthe concave/convex curvature of the eye-mountable device 210. In someexamples, the loop antenna 270 can be formed without making a completeloop. For instances, the antenna 270 can have a cutout to allow room forthe controller 250 and sensor electronics 260, as illustrated in FIG.2A. However, the loop antenna 270 can also be arranged as a continuousstrip of conductive material that wraps entirely around the flat surfaceof the substrate 230 one or more times. For example, a strip ofconductive material with multiple windings can be patterned on the sideof the substrate 230 opposite the controller 250 and sensor electronics260. Interconnects between the ends of such a wound antenna (e.g., theantenna leads) can then be passed through the substrate 230 to thecontroller 250.

When the eye-mountable device 210 is mounted over a corneal surface ofan eye, the motion of the eyelids distributes a tear film that coatsboth the concave and convex surfaces 224, 226. The tear film is anaqueous solution secreted by the lacrimal gland to protect and lubricatethe eye. The tear film layers coating the eye-mountable device 210 canbe about 10 micrometers in thickness and together account for about 10microliters. The eye-mountable device 210 may allow for electrodes to beexposed to the tear film via a channel in the polymeric material, orperhaps the polymeric material may be configured to allow for diffusionof tear film analytes to such sensor electrodes. For example, the sensorelectronics 260 may include one or more sensor electrodes of anamperometric analyte sensor, and a channel in the outward-facing convexsurface 224 may expose the sensor electrodes to a layer of tear fluidcoating the convex surface 224. The sensor electronics can then obtainan indication of an analyte concentration in the tear film by applying avoltage to the sensor electrodes and measuring a current through one orboth of the sensor electrodes. A suitable reagent can be fixed in thevicinity of the sensor electrodes to facilitate an electrochemicalreaction with a desired analyte. As the analyte is consumed by suchelectrochemical reactions, additional analyte diffuses to the sensor,and the rate of re-supply (i.e., the rate at which the analyte diffusesto the sensor) is related to the analyte concentration. The measuredamperometric current, which is related to the electrochemical reactionrate, is therefore indicative of the analyte concentration in the tearfilm.

III. Example Antenna Impedance Tuning System

FIG. 3 is a functional block diagram of an example system 300 configuredto adjust for temporal variations in antenna impedance. The system 300includes the antenna 310, an impedance measurement circuit 320, acontroller 330, an impedance tuning circuit 340, and a transceiver 350.The measurement circuit 320 is electrically connected to the antenna 310and functions to obtain a measurement indicative of the impedance of theantenna 310. The controller 330 is in communication with the measurementcircuit 320 and the tuning circuit 340. The controller 330 receives themeasurement from the measurement circuit 320, and determines whether andhow to adjust the tuning circuit 340 to cause the antenna 310 to beconnected to a conjugate matched impedance. The controller 330 theninstructs the tuning circuit 340 to make the determined adjustment. Thetransceiver 350 is electrically connected to the antenna 310 through thetuning circuit 340. By iteratively adjusting the tuning circuit 340 inaccordance with the measured impedance of the antenna 310 (as measuredby the measurement circuit 320), the controller 330 can cause theantenna-connected circuit components (e.g., the tuning circuit 340 andthe transceiver 350) to be conjugate matched to the antenna 310.

The antenna 310 can take a variety of different forms in differentapplications. The antenna 310 is a radiator with a body 312 formed of aconductive material that connects with the measurement circuit 320 andtuning circuit 340 via respective antenna leads 314, 316, which may beintegrally formed with the body 312. As shown in FIG. 3, the antenna 310may be a loop antenna, and may be similar in some respects to theantennas in body-mountable devices 100, 200 described above inconnection with FIGS. 1 and 2. For example, the antenna 310 may beformed by electroplated conductive material arranged in a loop. A loopantenna may be favorable in wireless power transfer applications, forexample. Moreover, while the antenna 310 is described as a radiator, insome examples the antenna 310 may be used in near field applications andfunction primarily reactively.

The impedance measurement circuit 320 functions to obtain a measurementindicative of the impedance of the antenna 310. In some examples, themeasurement circuit 320 may include a current source and a voltagesensor. The current source can convey a test current through the antennawhile the voltage sensor measures the voltage across the antenna 310 tosample the resulting waveform. For example, a voltage sensor may beimplemented by an amplifier connected across the antenna leads 314, 316.In some examples, the test current may also be measured using a currentmirror, which may be implemented using an active device that generates areplica current proportionate to the test current to allow formeasurement without loading the antenna 310. In one example, a currentmirror may be implemented by conveying the test current through a diodeconnected transistor, and then setting a base voltage (or gate voltage,etc.) of a “mirror” transistor based on a voltage that develops on thediode connected transistor. Other implementations are also possible togenerate a replica current for measurement purposes. The measurementcircuit 320 may also include a transimpedance amplifier for convertingthe replica current to voltage values for measurement purposes. Thus, atransimpedance amplifier may be connected across the current mirror andprovide voltage values to the controller 330 for measurement.

The controller 330 can be digital control module implemented by hardwarelogic to perform the functions described herein. The controller 330 canreceive indications of the measurements performed by the measurementcircuit 320, and use those measurements to determine adjustments to thetuning circuit 340. The controller 330 may include or be associated withan analog-to-digital converter for converting voltage values from themeasurement circuit 320 to digital values for further processing by thecontroller 330. For example, the voltage sensor in the measurementcircuit 320 may provide a voltage waveform to the analog-to-digitalconverter indicative of the measured voltage across the antenna leads314, 316 while the test current is being conveyed through the antenna310. In some examples, the analog-to-digital converter may also receivea voltage waveform from the transimpedance amplifier indicating thereplica current (and thus the test current) from the measurement circuit320. The analog-to-digital converter can then sample the waveforms at asampling frequency that depends on the frequency of the test currentsignal. In some cases, a multiplexer can be used to select between thetransimpedance amplifier and from the voltage sensor to enable samplingfrom each via the same analog-to-digital converter.

The controller 330 can then use the obtained measurements of the voltageacross the antenna 310, and the current conveyed through the antenna 310to extract the impedance of the antenna 310. In practice, the impedancemay be estimated using the relationship: Z=V/I, with Z the impedance, Vthe measured voltage, and I the test current. The signal may also beprocessed using I, Q down-conversion to separate orthogonal componentsof the voltage and/or current waveforms (e.g., real and imaginarycomponents). However, at least in some examples, the antenna 310 is notisolated from the tuning circuit 340 and transceiver 350 during themeasurements, and so the impedance estimated using the current andvoltage measurements is the impedance of the antenna 310 connected tothe tuning circuit 340 and transceiver 350. In another example, anadditional switch (or switches) may be used to isolate the antenna 310from the tuning circuit 340 during measurement. Such switches may besituated between the tuning circuit 340 and the connection of themeasurement circuit 320.

In examples in which the estimated impedance includes the tuning circuit340 and transceiver 350, the controller 330 can further determine theimpedance exhibited by the antenna 310. For example, the controller 330may be calibrated or otherwise configured with information indicative ofthe impedance of the transceiver 350. In addition, the controller 330may associate a particular impedance with the tuning circuit 340 basedon the current configuration thereof (e.g., the particular arrangementof reactive components connected to the antenna 310). In examples inwhich the transceiver 350 is implemented by an integrated circuit, thetransceiver impedance remains substantially stable over time and invarious environmental conditions. In practice, the controller 330 cantherefore associate a fixed impedance with the transceiver 350 and aconfiguration-dependent impedance with the tuning circuit 340. Thecontroller 330 can then determine the antenna impedance from theimpedance estimated based on the measurements by accounting for the two“known” impedances (of the transceiver 350 and tuning circuit 340) anddetermining the impedance attributable to the antenna 310.

Upon determining the antenna impedance, the controller 330 can thendetermine an adjustment to the tuning circuit 340 based on thedetermined antenna impedance, and instruct the tuning circuit 340 tomake the adjustment. For example, the controller 330 may determine aparticular arrangement of the selectively connected reactive elements inthe tuning circuit 340 and then provide suitable control signals toeffect the arrangement sought in the tuning circuit 340. For example,the antenna impedance may be determined to be Z_(ANT)=R_(ANT)+j X_(ANT),with R_(ANT) the resistive component, and X_(ANT) the reactivecomponent. The target impedance Z_(TARGET) of the combined tuningcircuit 340 and transceiver 350 is then Z_(TARGET)=R_(ANT)−j X_(ANT),and so the adjustments (if any) to the tuning circuit 340 are selectedto cause the tuning circuit 340 and transceiver 350 to approach thetarget impedance Z_(TARGET).

The adjustment to the tuning circuit 340 may cause the combined tuningcircuit 340 and transceiver 350 to have an impedance that is the complexconjugate of the determined antenna impedance (i.e., conjugate matched).In such a configuration the power transfer between transmitted/receivedradiation and the transceiver 350 is maximized. As a result, energyharvesting electronics operate with additional power, and communicationelectronics operate with greater signal margins and/or data rates. Insome examples, the adjustment to the tuning circuit 340 may beconfigured to account for the reactive component of the measuredimpedance, but not the resistive component. Such an approach may bewell-suited to applications in which the variability of the antennaimpedance is more susceptible to variations in the reactive componentthan the resistive component.

In some examples, the test current is a signal with a frequency bandnear about 915 megahertz (MHz), although other frequency bands may beselected, such as 13 MHz, 2.4 gigahertz (GHz), etc. In some cases,samples of the measured voltage(s) are obtained for a duration thatspans approximately 10 cycles of the signal waveform, although a varietyof different sampling durations may be used.

The impedance tuning circuit 340 includes one or more reactivecomponents that can be selectively connected to the antenna 310 (i.e.,to one or both of the leads 314, 316). In some examples, the tuningcircuit 340 may include a shunt capacitor configured to be connectedbetween the antenna leads 314, 316 via a switch. Additionally oralternatively, the tuning circuit 340 may include a shunt inductorconfigured to be connected between the antenna leads 314, 316 viaanother switch. Additionally or alternatively, the tuning circuit 340may include a series capacitor configured to be connected to one of theantenna leads 314, 316 via another switch (or pair of switches).Additionally or alternatively, the tuning circuit 340 may include aseries inductor configured to be connected to one of the antenna leads314, 316 via another switch (or pair of switches). Moreover, the tuningcircuit 340 may include multiple capacitors with different values thatcan be selectively connected in parallel (via respective switches) toprovide a total target shunt and/or series capacitance. And similarly,the tuning circuit 340 may include multiple inductors with differentvalues that can be selectively connected (via respective switches) toprovide a total desired shunt and/or series inductance. The variousswitches that connect the reactive components in the impedance tuningcircuit 340 may be implemented by transistors that operate in accordancewith control signals from the controller 330. Using the switches, thecontroller 330 can selectively connect and/or disconnect the reactivecomponents to/from the antenna 310 so as to make the desired impedanceadjustment.

Additionally or alternatively, the impedance tuning circuit 340 mayinclude one or more reactive components with reactances that can beadjusted via the controller 330. For example, the impedance tuningcircuit 340 may include a reactive component with a capacitance and/orinductance that depends on an applied voltage, and the controller 330may regulate the reactance by adjusting the voltage applied to such acomponent. In some examples, such a variable capacitor (e.g., avaractor) may be connected to one or both leads 314, 316 of the antenna310. Other examples of adjustable reactive components included in theimpedance tuning circuit 340 and regulated by the controller 330 so asto create an impedance that is conjugate matched to the antenna 310 arealso possible.

The transceiver 350 may be implemented in a number of different ways.The transceiver 350 may include communication circuitry configured tosend and receive wireless communications using the antenna by modulatingphase, frequency, and/or amplitude of carrier signals so as to encodedata. In some examples, the transceiver 350 can additionally oralternatively include energy harvesting circuitry, such as a rectifierand regulator, which power electronics components, similar to thedescription of energy harvesting systems described above in connectionwith the body-mountable devices 110, 210 of FIGS. 1 and 2.

In an example operation, the controller 330 can function to cause theantenna 310 to be connected to a conjugate matched impedance, even asthe impedance of the antenna 310 undergoes variations, by making realtime adjustments to the impedance tuning circuit 340 in a manner thataccounts for those variations. The system 300 may be used in anenvironment with conditions that lead to temporal variations in antennaimpedance (e.g., variations in dielectric loading, temperature,humidity, etc.). For example, the system 300 may be included in abody-mountable device, such as the body-mountable devices 110, 210discussed above in connection with FIGS. 1 and 2. Thus, in someexamples, the system 300 may be situated on a substrate embedded withina biocompatible polymeric material, which may be formed to include abody-mountable surface.

FIG. 4A is a functional block diagram of an example system 400configured to adjust for temporal variations in antenna impedance. Thesystem 400 is similar in some respects to the system 300 described inconnection with FIG. 3 and may be considered an example implementationof the system 300. The system 400 includes an antenna 410, an impedancemeasurement circuit 420, a controller 430, an impedance tuning circuit440, and a transceiver 450. The antenna 410 includes a conductiveradiative body 412, which may be configured as a loop, and antenna leads414, 416 through which the antenna 410 connects to the remaining circuitcomponents.

The impedance measurement circuit 420 includes a current source 424connected to convey a test current through the antenna 410. A currentmirror 426 generates a replica current based on the test current. Thereplica current from the current mirror 426 provides a current that canbe sampled for measurement without electrically loading the antenna 410.To facilitate such measurements, the current mirror 426 is connected toa transimpedance amplifier 428, which outputs a voltage that depends onthe replica current and thus converts the replica current to a voltagesignal that can be measured. The current mirror 426 may be implementedby a range of different technologies based on the desired frequencyresponse, size/power limitations, etc. The measurement circuit 420 alsoincludes a voltage sensor 422, which may be an amplifier connectedacross the antenna leads 414, 416.

Both the output of the voltage sensor 422 and the output of thetransimpedance amplifier 428 are connected to inputs of a multiplexer432, which selects one of the voltage signals to provide to ananalog-to-digital converter (ADC) 434 based on instructions from thecontroller 430. Thus, the controller 430 can instruct the multiplexer432 to select one of the measurement sources (i.e., the transimpedanceamplifier 428 or the voltage sensor 422), which causes the ADC 434 tosample the selected voltage source. The controller 430 can then receivea series of digital values from the ADC 434 as the ADC samples 434 theincoming voltage waveform. The controller 430 can then analyze thewaveform(s) of the test current conveyed through the antenna 410 (fromthe transimpedance amplifier 428) and the resulting voltage across theantenna 410 (from the voltage sensor 422), and determine the antennaimpedance. Similar to the description of controller 330 provided abovein connection with FIG. 3, the controller 430 determining the antennaimpedance may involve associating a fixed impedance with the transceiver450 and a configuration-dependent impedance with the tuning circuit 440(the “known” impedances). The controller 430 may then determine theimpedance to attribute to the antenna 410 based on the impedance derivedfrom the series of measurements by accounting for the two “known”impedances.

The controller can then determine an adjustment to the tuning circuit440 that matches the determined antenna impedance. The controller 430can cause the tuning circuit 440 to make the determined adjustment byproviding suitable control signals 436 to switches in the tuning circuit440 that cause reactive elements therein to be connected to the antenna410. As shown in FIG. 4A, the tuning circuit 440 includes multiple shuntcapacitors 442, 444, 446 each arranged to be connected across theantenna leads 414, 416 by a respective switch 441, 443, 445, which alloperate in accordance with the signals 436 from the controller 430. Eachof the capacitors 442, 444, 446 may have a different value such thatconnecting different combinations can provide a different total shuntcapacitance, and thus a different impedance adjustment. In one example,capacitor 446 may have the least capacitance C, and capacitor 444 mayhave a capacitance of 2 C, and each additional capacitor can have acapacitance that is double the next-smallest one. Capacitor 442 may havea capacitance of 2^(N-1) C, with N the total number of shunt capacitorsin the tuning circuit 440. Selectively connecting such a bank ofcapacitors in different combinations can allow for the total shuntcapacitance to be selected in increments of C. Other arrangements ofselectively connected reactive elements in shunt and/or seriesarrangements are also possible as noted above in connection with FIG. 3.

FIG. 4B is a functional block diagram of an example system 401configured to adjust for temporal variations in antenna impedance. Thesystem 401 is similar in some respects to the system 300 described inconnection with FIG. 3 and may be considered an example implementationof the system 300. In addition to the antenna 410 and the transceiver450, which have been described above in connection with FIG. 4A, thesystem 401 includes a directional coupler 460, an impedance measurementcircuit 470, a controller 480, and an impedance tuning circuit 490. Thesystem 401 functions to obtain a measurement of the antenna impedanceusing the impedance measurement circuit 470, determine an adjustment tothe impedance tuning circuit 490 via the controller 480, and cause theimpedance tuning circuit 490 to make the determined adjustment. However,unlike the system 400 described in connection with FIG. 4A, the system401 obtains a measurement of the antenna impedance by measuring both atest signal applied to the antenna 410 and a reflected signal returningfrom the antenna 410, and then determining the antenna impedance usingboth measurements and a reference impedance.

The directional coupler 460 is a three port device that conveys signalsbetween an input port 462 and a transmitted port 464 and a coupled port466. In practice, signals received at the input port 462 are dividedbetween both the transmitted port 464 and the coupled port 466, with therelative power allocated to each based on the configuration of thedirectional coupler 460. In addition, signals received at thetransmitted port 464 are conveyed to the input port 462. However, thecoupled port 466 and the transmitted port 464 are substantially isolatedfrom one another, and so signals input to the transmitted port 464 arenot conveyed to the coupled port 466 or vice versa. Thus, thedirectional coupler 460 functions to cause signals originating at theantenna transceiver 450 and passing through the impedance tuning circuit490 to reach through the antenna 410 (e.g., by entering the transmittedport 464 and exiting the input port 462). Whereas signals from theantenna 410 (e.g., due to received radiated energy and/or reflectedenergy from signals applied to the antenna 410) enter the input port 462and are conveyed both to the transceiver 450 (via transmitted port 464)and also to the impedance measurement circuit 470 (via coupled port466). It is noted that in some embodiments, the directional coupler 460may be implemented as a four-port device, such as a microwave waveguidethat is substantially symmetrical. For example, a second coupled portmay receive a portion of signals input to the transmitted port 464, andmay be substantially isolated from the input port 462. In someembodiments, such an additional coupled port may be terminated by aresistor so as to absorb any incoming signals and dampen any reflectionsfrom returning into the coupler 460.

The impedance measurement circuit 470 includes a test signal measurementcircuit 472 and a reflected signal measurement circuit 474. The testsignal measurement circuit 472 is configured to measure a signalstrength (e.g., a measurement of voltage and/or current magnitude) of atest signal applied to the antenna 410 (e.g., a test signal from thetransceiver 450 conveyed via the impedance tuning circuit 490). Thus,the test signal measurement circuit 472 can be electrically coupledbetween the impedance tuning circuit 490 and the transmitted port 464 ofthe directional coupler 460. The reflected signal measurement circuit474 is configured to measure a signal strength (e.g., a measurement ofvoltage and/or current magnitude) of a reflected signal returning fromthe antenna 410 (e.g., a reflected signal from the antenna 410 thatresults from the application of the test signal). Thus, the reflectedsignal measurement circuit 474 can be electrically coupled to thecoupled port 466 of the directional coupler 460 so as to receive atleast a portion of signals coming from the antenna 410. Each of themeasurement circuits 472, 474 may include voltage sensors such as anamplifier that outputs a signal, such as a voltage value, to thecontroller 480, which can then analyze the measurements digitally via ananalog to digital converter. To facilitate understanding, the path ofthe applied test signal through the directional coupler 460 (i.e., fromthe transmitted port 464 to the input port 462) is indicated by adirectional arrow 468 labeled “TEST.” Similarly, the path of thereflected signal through the directional coupler 460 (i.e., from theinput port 462 to the coupled port 466) is indicated by a directionalarrow 469 labeled “REFLECTED.”

The controller 480 receives measurements from both the test signalmeasurement circuit 472 and reflected signal measurement circuit 474,and uses those measurements in combination with a predeterminedcharacteristic impedance of the directional coupler 460 to determine theimpedance of the antenna 410. In some examples, the controller 480 maydetermine the antenna impedance by first determining the reflectioncoefficient, F, of the antenna 410 and directional coupler 460considered together, and then solving for the antenna impedance. Thereflection coefficient F may be determined from the obtainedmeasurements of the magnitudes of the applied test signal and thereflected signal, as indicated by the relationship below, in which|Vtest| is the magnitude of the voltage of the applied test signal,|Vreflected| is the magnitude of the voltage of the reflected signal,Zant is the impedance of the antenna 410, and Zcoupler is thecharacteristic impedance of the directional coupler 460.

$\Gamma = {\frac{{Vreflected}}{{Vtest}} = \frac{{Zant} - {Zcoupler}}{{Zant} + {Zcoupler}}}$

Thus, to determine the antenna impedance, Zant, the controller 480 canobtain measurements of |Vtest| and |Vreflected| (from the test signalmeasurement circuit 472 and reflected signal measurement circuit 474,respectively), calculate F, and then solve the relation for Zant using apredetermined value for Zcoupler. Once the controller 480 determines theantenna impedance (i.e., Zant), the controller 480 can then determine anappropriate adjustment to the impedance tuning circuit 490 that willcause the antenna 410 to be coupled to a conjugate matched impedance,similar to the discussion of the controller 430 described above inconnection with FIG. 4A.

In some examples, the electrical connections between the directionalcoupler 460, the antenna 410, and the impedance measurement circuit 470may use shielded connections such that the impedance of thoseconnections are not influenced by dielectric loading in the vicinity ofthe system 401. As such, the system 401 can function to obtain ameasurement of the impedance of the antenna 410, which may be due tovariations in dielectric loading, temperature, and/or other factors thatchange over time.

The impedance tuning circuit 490 can include one or more reactivecomponents that can be selectively connected to the antenna 410 (via thedirectional coupler 460) and/or that have an adjustable impedance valuesuch that the reactive components can be used to adjust the impedancecoupled to the antenna 410. Thus, the impedance tuning circuit 490 mayinclude a combination of series connected and/or shunt connectedinductors and/or capacitors spanning a range of values that can each beselectively connected to the antenna 410 according to control signalsfrom the controller 480. Further, the impedance tuning circuit 490 mayinclude one or more variable impedance components, such as a varactor ora similar component that can adjust its impedance based on an inputsignal regulated by the controller 480. Of course, other arrangements ofselectively connected reactive elements in shunt and/or seriesarrangements are also possible as noted above in connection with FIG. 3.

IV. Example Reader and Eye-Mountable Device System

FIG. 5A is a block diagram of a system 500 with an eye-mountable device530 and an external reader 510. FIG. 5B is a block diagram of theeye-mountable device 530 shown in FIG. 5A. The eye-mountable device 530is configured to be contact-mounted over a corneal surface of an eye 10.

The eye-mountable device 530 includes a communication system and/orenergy harvesting system with an antenna 532 and an impedance tuningsystem 534, similar to the impedance tuning systems described herein inconnection with FIGS. 1-4. Thus, the impedance tuning system 534 canfunction to obtain measurements indicating the impedance of antenna 532,and selectively connect reactive components to the antenna 532 so as toincrease power transfer to/from the antenna 532 (e.g., by connecting theantenna 532 with an impedance matched circuit). The eye-mountable device530 also includes power/data electronics 536 which are electricallyconnected to the antenna 532 through the impedance tuning system 534.The power/data electronics 536 may harvest energy from incidentradiation 520 received at the antenna 532 and provide power tobio-interactive electronics 538. The bio-interactive electronics 538 mayperform a variety of functions such as measuring an analyteconcentration. In some cases, the power/data electronics 536 can use theantenna 532 to communicate information from the bio-interactiveelectronics 538 (e.g., a sensor measurement) to the reader 510 viabackscatter radiation 522.

The external reader 510 includes an antenna 512 and an impedance tuningsystem 514, similar to the impedance tuning systems described herein inconnection with FIGS. 1-4. Thus, the impedance tuning system 514 canfunction to obtain measurements indicating the impedance of antenna 512,and selectively connect reactive components to the antenna 512 so as toincrease power transfer to/from the antenna 512 (e.g., by connecting theantenna 512 with a conjugate matched circuit). The external reader 510can also include a processing system 516 and a memory 518. Theprocessing system 516 can be a computing system that executes softwarestored in the memory 518 to cause the system 500 to operate as describedherein to obtain information from the eye-mountable device (e.g., sensormeasurements obtained using the bio-interactive electronics 538). Inpractice, the reader 510 may query the eye-mountable device 530 byintermittently transmitting radio frequency 520 to power theeye-mountable device 530, and then receive an indication of ameasurement in backscatter radiation 522.

By including impedance tuning systems both in the eye-mountable device530 and the external reader 510, the wireless link for both powertransfer and data transfer is improved from both sides. The reader 510may be incorporated into a head-mountable system, an article of clothingor jewelry, or another article configured to be worn near a user's eyeor head. In one example, which is illustrated further by FIGS. 6A-6C,the reader may be integrated into an eyeglasses frame configured to beworn on a user's face.

FIG. 6A illustrates an example head-mountable eyeglass frame 600. Theeyeglass frame 600 includes end pieces 602-604, pads 606-608, andeyepiece sections 610-612. A loop antenna 614 is disposed along aperiphery of the eyepiece section 610. The eyeglass frame 600 alsoincludes impedance tuning circuitry 616 and a power supply 618, andprocessing system.

The end pieces 602-604 can be formed from any material (e.g., plastic,metal, composite material, etc.) suitable for supporting the componentsof the eyeglass frame 600. In some examples, the end pieces 602-604 canbe shaped to correspond with a wearer's ears such that the eyeglassframe 300 can be comfortably mounted to the wearer's head. Additionally,the pads 606-608 can be formed from similar materials suitable forsupporting the eyeglass frame 600 and the included components and shapedto mount on the wearer's nose.

Although not illustrated in FIG. 6A, in some examples, the arrangementof the eyeglass frame 600 can omit the end pieces 602-604 and/or thepads 606-608. For example, the eyeglass frame 600 can be implementedwithout the end pieces 602-604. In this example, the pads 606-608 cansupport the eyepiece sections 610-612 and other components of theeyeglass frame 600 when the eyeglass frame 600 is mounted to the head.In examples, the eyepiece sections 610-612 can be supported by anelastic band or other means.

The eyepiece sections 610-612 can be shaped to allow an environment ofthe wearer to be viewable through a central area of the eyepiecesections 610-612. For example, the eyepiece section 610 may support atransparent material such as a lens or other optical element in thecentral area such that the environment is viewable through the centralarea. Alternatively, the transparent material may be omitted such thatthe central area of the eyepiece sections 610-612 allows lightpropagating towards the wearer's eye to travel through the central area.

The loop antenna 614 is disposed along a periphery of the eyepiecesection 610 such that the loop antenna 614 does not obstruct the view ofthe wearer when the eyeglass frame 600 is mounted to the wearer's head.In some examples, the loop antenna 614 can be implemented as a wire ofconductive material that is shaped in accordance with a shape of theperiphery of the eyepiece section 610 as illustrated in FIG. 6A. In someexamples, the loop antenna 614 can be at least partially embedded in theperiphery of the eyepiece section 610. Moreover, the loop antenna 614can include multiple loops wound along the periphery of the eyepiecesection 614 in parallel and/or coiled in series.

The loop antenna 614 can be configured to receive an electric currentand transmit electromagnetic energy. In some examples, a link efficiencyof the loop antenna 614 (or “link gain”) can be based, at least in part,on the transmission frequency of the loop antenna 614. In one example,if the transmission frequency corresponds to a resonant frequency of theloop antenna, the link efficiency can be increased. Thus, in thisexample, a high portion of the energy in the input electric current canbe converted to the electromagnetic energy. Additionally oralternatively, the link efficiency of the loop antenna 614 can beimproved by impedance matching between the loop antenna 614 and theinput signal from the power supply 618.

To facilitate adjusting the resonant frequency of the loop antenna 614or the input impedance of the loop antenna 614, the eyeglass frame 600can optionally include the tuning circuitry 616. The tuning circuitry616 can include reactive tuning components, such as inductors andcapacitors, arranged to adjust the input impedance of the loop antenna614 and/or tune the resonant frequency of the loop antenna 614, similarto the description of impedance tuning systems above. In some examples,the tuning circuitry 316 can include suitable devices (e.g., inductors,capacitors, etc.) arranged to modify the resonance frequency of the loopantenna 314 (e.g., LC tuning).

FIG. 6B illustrates the head-mountable eyeglass frame 600 of FIG. 6Amounted to a head 620. As illustrated in FIG. 6B, the end pieces 602-604are mounted, respectively, on ears 622-624 to support the eyeglass frame600 on the wearer's head 620. So worn, the wearer is able to view thesurrounding environment through the eyepiece sections 610, 612.

FIG. 6C is a close-in view enhanced to show an eye-mountable device 630mounted on a cornea 626 and the loop antenna 614 disposed on thehead-mountable eyeglass frame 600 of FIGS. 6A-6B. The structure andfunction of the eye-mountable device 630 can be similar to theeye-mountable device 110 in FIG. 1 and the eye-mountable device 210 inFIGS. 2A-2B, and the eye-mountable device 530 in FIGS. 5A-5B. Forexample, the eye-mountable 630 may include electronic componentssuitable for wirelessly communicating with the loop antenna 614 and forharvesting power from electromagnetic energy transmitted by the loopantenna 614.

As illustrated in FIG. 6C, when the eyeglass frame 600 is mounted to thehead 620, the loop antenna 614 is at a short distance from theeye-mountable device 630. Thus, for example, if the short distance iswithin a near-field of the loop antenna 614, a reactive component of theelectromagnetic energy transmitted by the loop antenna can be harvestedby the eye-mountable device 630. Further, for example, a radiativecomponent of the electromagnetic energy (e.g., RF radiation) can also beharvested with a small loss due to the short distance. Additionally,relative motion between the loop antenna 614 and the eye-mountabledevice 630 is minimal as the head 620 moves. Thus, a high efficiency andlow variability of power transfer from the loop antenna 614 to theeye-mountable device 630 may be realized in the illustrated embodiment.

In addition, as the impedance of the loop antenna 614 changes over time,those changes can be accounted for using the impedance tuning circuitry616. Such changes in impedance of the loop antenna 614 may occur, forexample, due to changes in the dielectric loading from the wearer's head620 (e.g., due to changes in position on the wearer's head 620, changingdistributions of perspiration and/or tear fluid, etc.). Similarly, asthe impedance of the antenna within the eye-mountable device 630 changesover time, those changes can be accounted for using the impedance tuningcircuitry included in the eye-mountable device 630. Such changes in theimpedance of the antenna in the eye-mountable device 630 may occur, forexample, due to changes in the dielectric loading from the wearer'scornea 626 (e.g., due to changes in position on the corneal surface,variable corneal thickness, changes in tear fluid distribution, etc.).As a result, wireless power provided to the eye-mountable device 630,from the eyeglass frame 600 is transmitted at a relatively highefficiency from the loop antenna 614. And that radiation is received atthe eye-mountable device 630 by its loop antenna, and the power istransferred to the energy harvesting systems therein, with a relativelyhigh efficiency.

V. Example Operations

FIG. 7 is a flowchart of an example process 700 involving adjusting animpedance tuning circuit based on a real time measurement of theimpedance of an antenna. The process 700 may be performed using any ofthe impedance tuning systems described herein in connection with FIGS.1-6. For example purposes, some functions in process 700 are describedin connection with the system 300 of FIG. 3.

At block 702, an impedance measurement circuit is used to obtain ameasurement indicative of an impedance of an antenna that is coupled toan impedance tuning circuit. For example, the impedance measurementcircuit 320 may convey a test current through antenna 310, and measurethe voltage across the antenna 310 while the test current is beingconveyed. The measured voltage, in combination with the test current,can then be used to estimate the antenna impedance. In some examples,the obtained measurement may also include a measurement of the testcurrent. The measurement(s) can then be provided to the controller 330,(e.g., via an analog-to-digital converter) for processing.

At block 704, a controller determines an adjustment to the impedancetuning circuit based on the obtained measurement. For example, thecontroller 330 can determine the impedance of the antenna 310, and thendetermine a particular arrangement with which to connect reactiveelements in the tuning circuit 340 such that the antenna 310 is matchedto its complex conjugate impedance. As describe above, the controller310 determining the adjustment may involve accounting for a fixedimpedance associated with transceiver and/or energy harvestingcomponents and may involve accounting for a configuration-dependentimpedance associated with the tuning circuit 320.

At block 706, the controller can cause the impedance tuning circuit tomake the determined adjustment. For example, the controller 330 cangenerate suitable control signals to cause switches in the tuningcircuit 340 to selectively connect (or disconnect) particular reactiveelements to (or from) the antenna 310.

In addition, the process 700 may be an iterative process that isperformed to iteratively update the impedance tuning circuit inaccordance with variations in the antenna impedance.

Moreover, it is particularly noted that while the body-mountableelectronics platform is described herein by way of example as aneye-mountable device or an ophthalmic device, it is noted that thedisclosed systems and techniques can be applied in other contexts aswell. For example, contexts in which electronics platforms are operatedwith low power budgets (e.g., via harvested energy from radiatedsources) or are constrained to small form factors (e.g., implantablebio-sensors or other wearable electronics platforms) may employ thesystems and processes described herein to optimize their wireless powerand/or data links. In one example, an implantable medical device may beencapsulated in biocompatible material and implanted within a hostorganism. The implantable medical device may include a circuitconfigured to wireless communicate and/or receive power via an antennaand an impedance tuning system configure to adjust the impedanceconnected to the antenna based on real time measurements of the antennaimpedance.

FIG. 8 depicts a computer-readable medium configured according to anexample embodiment. In example embodiments, the example system caninclude one or more processors, one or more forms of memory, one or moreinput devices/interfaces, one or more output devices/interfaces, andmachine-readable instructions that when executed by the one or moreprocessors cause the system to carry out the various functions, tasks,capabilities, etc., described above.

As noted above, in some embodiments, the disclosed techniques can beimplemented by computer program instructions encoded on a non-transitorycomputer-readable storage media in a machine-readable format, or onother non-transitory media or articles of manufacture (e.g., in thesystem 100, such non-transitory media may include instructions 184stored on the memory storage 182 of the external reader 180, orinstructions stored on the body-mountable device 110 and performed bythe controller 150). FIG. 8 is a schematic illustrating a conceptualpartial view of an example computer program product that includes acomputer program for executing a computer process on a computing device,arranged according to at least some embodiments presented herein.

In one embodiment, the example computer program product 800 is providedusing a signal bearing medium 802. The signal bearing medium 802 mayinclude one or more programming instructions 804 that, when executed byone or more processors may provide functionality or portions of thefunctionality described above with respect to FIGS. 1-7. In someexamples, the signal bearing medium 802 can be a non-transitorycomputer-readable medium 806, such as, but not limited to, a hard diskdrive, a Compact Disc (CD), a Digital Video Disk (DVD), a digital tape,memory, etc. In some implementations, the signal bearing medium 802 canbe a computer recordable medium 808, such as, but not limited to,memory, read/write (R/W) CDs, R/W DVDs, etc. In some implementations,the signal bearing medium 802 can be a communications medium 810, suchas, but not limited to, a digital and/or an analog communication medium(e.g., a fiber optic cable, a waveguide, a wired communications link, awireless communication link, etc.). Thus, for example, the signalbearing medium 802 can be conveyed by a wireless form of thecommunications medium 810.

The one or more programming instructions 804 can be, for example,computer executable and/or logic implemented instructions. In someexamples, a computing device is configured to provide variousoperations, functions, or actions in response to the programminginstructions 804 conveyed to the computing device by one or more of thecomputer readable medium 806, the computer recordable medium 808, and/orthe communications medium 810.

The non-transitory computer readable medium 806 can also be distributedamong multiple data storage elements, which could be remotely locatedfrom each other. The computing device that executes some or all of thestored instructions could be an external reader, such as the reader 180illustrated in FIG. 1, or another mobile computing platform, such as asmartphone, tablet device, personal computer, etc. Alternatively, thecomputing device that executes some or all of the stored instructionscould be remotely located computer system, such as a server.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopebeing indicated by the following claims.

What is claimed is:
 1. A system comprising: an antenna; an impedancemeasurement circuit electrically coupled to the antenna, wherein theimpedance measurement circuit is configured to be used to obtain ameasurement indicative of an impedance of the antenna; an impedancetuning circuit electrically coupled to the antenna, wherein theimpedance tuning circuit includes one or more reactive elements that canbe used to adjust the impedance coupled to the antenna; and a controllerconfigured to: (i) use the impedance measurement circuit to obtain ameasurement indicative of an impedance of the antenna; (ii) determine anadjustment to the impedance tuning circuit based on the obtainedmeasurement; and (iii) cause the impedance tuning circuit to make thedetermined adjustment.
 2. The system of claim 1, further comprising: atransceiver connected to the antenna through the impedance tuningcircuit, wherein the transceiver is configured to use the antenna tosend and receive signals, and wherein the transceiver has apredetermined impedance; and wherein the controller is configured todetermine the adjustment to the impedance tuning circuit by a processcomprising: (i) estimating an impedance of the antenna based on theobtained measurement; (ii) determining a target impedance of theimpedance tuning circuit based on the predetermined impedance of thetransceiver and the estimated impedance of the antenna; and (iii)determining one or more adjustments to the impedance tuning circuitbased on the target impedance.
 3. The system of claim 1, wherein theimpedance measurement circuit comprises: (i) a test current sourceconfigured to convey a test current through the antenna, and (ii) avoltage sensor configured to measure a voltage across the antenna whilethe test current is conveyed through the antenna; and wherein thecontroller is configured to receive from the voltage sensor a series ofvoltage measurements of voltage across the antenna, wherein the seriesof voltage measurements have a sampling frequency corresponding to afrequency of the test current.
 4. The system of claim 3, wherein theimpedance tuning circuit includes one or more switches that selectivelycouple the one or more reactive elements to the antenna, and wherein thecontroller is further configured to: (i) use the series of voltagemeasurements and an indication of the test current to determine acombined impedance of the antenna and the impedance tuning circuit; (ii)determine a state-dependent impedance of the impedance tuning circuitbased on a state of the one or more switches during the measurement; and(iii) estimate the impedance of the antenna based on the combinedimpedance and the state-dependent impedance.
 5. The system of claim 1,wherein the impedance measurement circuit comprises: (i) a test currentsource configured to convey a test current through the antenna, (ii) avoltage sensor configured to measure a voltage across the antenna whilethe test current is conveyed through the antenna, and (iii) a currentmirror configured to provide a replica current that mirrors the testcurrent conveyed through the antenna; and wherein the controller isconfigured to use the impedance measurement circuit to obtain themeasurement by a process comprising (i) obtaining from the voltagesensor a series of measurements of voltage across the antenna while thetest current is conveyed through the antenna, and (ii) obtaining aseries of measurements of the replica current provided by the currentmirror.
 6. The system of claim 1, wherein the controller is configuredto make the determined adjustment via one or more switches thatselectively couple the one or more reactive elements to the antenna, andwherein the impedance tuning circuit is electrically coupled to theantenna via antenna leads, and wherein the one or more reactive elementsinclude at least one of: a shunt capacitor coupled across the antennaleads via at least one of the one or more switches; a series capacitorcoupled in series with one of the antenna leads via at least one of theone or more switches; a shunt inductor coupled across the antenna leadsvia at least one of the one or more switches; or a series inductorcoupled in series with one of the antenna leads via at least one of theone or more switches.
 7. The system of claim 1, wherein the controlleris configured to make the determined adjustment via one or more switchesthat selectively couple the one or more reactive elements to theantenna, and wherein the one or more reactive elements include multipleshunt capacitors having respective capacitances, wherein each of themultiple capacitors is coupled across leads of the antenna via arespective one of the one or more switches.
 8. The system of claim 1,further comprising: a polymeric material formed to include abody-mountable surface; and a substrate at least partially embeddedwithin the polymeric material, wherein the antenna, the impedancemeasurement circuit, the impedance tuning circuit, and the controllerare disposed on the substrate.
 9. The system of claim 8, wherein thepolymeric material has a concave surface and a convex surface, whereinthe concave surface is configured to be removably mounted over a cornealsurface and the convex surface is configured to be compatible witheyelid motion when the concave surface is so mounted.
 10. A methodcomprising: obtaining a measurement indicative of an impedance of anantenna; determining, based on the obtained measurement, an adjustmentto an impedance tuning circuit coupled to the antenna, wherein theimpedance tuning circuit includes one or more reactive elements that canbe used to adjust the impedance coupled to the antenna; and causing theimpedance tuning circuit to make the determined adjustment.
 11. Themethod of claim 10, further comprising: estimating an impedance of theantenna based on the obtained measurement; determining a targetimpedance of the impedance tuning circuit based on the estimatedimpedance of the antenna and a predetermined impedance of a transceivercoupled to the antenna through the impedance tuning circuit; anddetermining one or more adjustments to the impedance tuning circuitbased on the target impedance.
 12. The method of claim 10, whereinobtaining the measurement comprises: conveying a test current throughthe antenna; and measuring a voltage across the antenna while the testcurrent is conveyed through the antenna, wherein the obtainedmeasurement comprises the measured voltage.
 13. The method of claim 12,wherein obtaining the measurement further comprises: obtaining a seriesof voltage measurements of voltage across the antenna while the testcurrent is conveyed through the antenna, wherein the series of voltagemeasurements have a sampling frequency corresponding to a frequency ofthe test current.
 14. The method of claim 13, wherein the one or morereactive elements can be selectively coupled to the antenna via one moreswitches, the method further comprising: using the series of voltagemeasurements and an indication of the applied test current to determinea combined impedance of the antenna and the impedance tuning circuit;determining a state-dependent impedance of the impedance tuning circuitbased on a state of the one or more switches; and estimating theimpedance of the antenna based on the combined impedance and thestate-dependent impedance.
 15. The method of claim 10, wherein obtainingthe measurement comprises: obtaining from the voltage sensor a series ofvoltage measurements of voltage across the antenna while the testcurrent is conveyed through the antenna; and obtaining a series ofcurrent measurements of a replica current provided by a current mirror,wherein the replica mirrors the test current conveyed through theantenna.
 16. A body-mountable device comprising: a polymeric materialformed to include a body-mountable surface; a substrate at leastpartially embedded within the polymeric material; an antenna disposed onthe substrate; an impedance measurement circuit disposed on thesubstrate, wherein the impedance measurement circuit is coupled to theantenna, wherein the impedance measurement circuit is configured to beused to obtain a measurement indicative of an impedance of the antenna;an impedance tuning circuit disposed on the substrate, wherein theimpedance tuning circuit is coupled to the antenna, wherein theimpedance tuning circuit includes one or more reactive elements that canbe used to adjust the impedance coupled to the antenna; and a controllerdisposed on the substrate, wherein the controller is configured to: (i)use the impedance measurement circuit to obtain a measurement indicativeof an impedance of the antenna; (ii) determine an adjustment to theimpedance tuning circuit based on the obtained measurement; and (iii)cause the impedance tuning circuit to make the determined adjustment.17. The body-mountable device of claim 16, further comprising: atransceiver disposed on the substrate, wherein the transceiver isconnected to the antenna through the impedance tuning circuit, whereinthe transceiver is configured to use the antenna to send and receivesignals, and wherein the transceiver has a predetermined impedance; andwherein the controller is configured to determine the adjustment to theimpedance tuning circuit by a process comprising: (i) estimating animpedance of the antenna based on the obtained measurement; (ii)determining a target impedance of the impedance tuning circuit based onthe predetermined impedance of the transceiver and the estimatedimpedance of the antenna; and (iii) determining one or more adjustmentsto the impedance tuning circuit based on the target impedance.
 18. Thebody-mountable device of claim 16, wherein the impedance measurementcircuit comprises: (i) a test current source configured to convey a testcurrent through the antenna, and (ii) a voltage sensor configured tomeasure a voltage across the antenna while the test current is conveyedthrough the antenna; and wherein the controller is configured to receivefrom the voltage sensor a series of voltage measurements of voltageacross the antenna, wherein the series of voltage measurements have asampling frequency corresponding to a frequency of the test current. 19.The body-mountable device of claim 18, wherein the impedance tuningcircuit includes one or more switches that selectively couple the one ormore reactive elements to the antenna, and wherein the controller isfurther configured to: (i) use the series of voltage measurements and anindication of the test current to determine a combined impedance of theantenna and the impedance tuning circuit; (ii) determine astate-dependent impedance with the impedance tuning circuit based on astate of the one or more switches during the measurement; and (iii)estimate the impedance of the antenna based on the combined impedanceand the state-dependent impedance.
 20. The body-mountable device ofclaim 16, wherein the polymeric material has a concave surface and aconvex surface, wherein the concave surface is configured to beremovably mounted over a corneal surface and the convex surface isconfigured to be compatible with eyelid motion when the concave surfaceis so mounted.